A characteristic of CANDU® steam generator tubes is that they have protective deposits of magnetite on the internal diameter (ID) surfaces. There is currently no method that accurately and reliably measures the thickness of magnetite deposited on the ID of steam generator tubing.
Probes for inspecting the inner walls of metallic conduits are known in the prior art. Such probes are particularly useful in inspecting the internal walls of heat exchangers in nuclear steam generators for flaws or deformation caused by corrosion, fretting, or the accumulation of sludge products in the crevice regions of the generator. Generally, these probes operate by means of either strain gauges, or eddy current probes.
Strain gauge-type probes are generally formed from a cylindrical mandrel that is circumscribed by a plurality of the spring-loaded fingers. Strain gauges are placed onto each of the spring-loaded fingers. When the probe body is inserted into the interior of a tube and translated along its longitudinal axis, differences in the radius of the internal tube walls cause one or more of the spring fingers to flex in a radial direction. The extent to which these fingers flex is picked up by the strain gauges attached to the fingers.
Eddy current probes are generally formed by an eddy current coil resiliently mounted in a probe head so as to wipingly engage the interior of the tube being inspected when the probe is rotated. The coil is electrically connected to a current generator which conducts an alternating current to the coil as it is moved. An impedance detecting circuit is also connected across the leads of the coil. In operation, the alternating current conducted through the coil excites it into generating a pulsating magnetic field whose magnitude and polarity changes in accordance with the frequency of the current. When the coil of the probe is positioned in the vicinity of an electrically conductive wall, the changing magnetic flux emanating from the coil induces eddy currents in a portion of the wall. The particular amperage, voltage and direction of the eddy currents produced are dependent in part upon the specific impedance of the portion of the wall that conducts the eddy current. Because the direction of flow of the eddy currents generated by the coil is opposite to the current flowing through the probe sensing coil, the magnetic field created by the eddy currents creates an impedance in the sensing coil. The strength of these eddy currents is in turn dependent upon the resistance that these currents encounter as they circulate through the wall. Since flaws in the metal wall (such as cracks, pits or regions of local thinning) create regions of higher resistances at flaw locations, eddy current probes can be used to locate flaws by constantly monitoring the impedances of the sensing coils as the probe body is moved along the internal walls of the tube.
While some prior art probes are capable of performing satisfactory inspections of heat exchanger tubes, they each suffer from drawbacks that have limited their usefulness. In addition, these probes do not permit measurement of a deposit on the ID surfaces.
Strain gauge-type probes tend to be delicate since they require the mounting of very small strain gauges onto the resilient metal fingers that circumscribe the probe body. Both the strain gauges themselves and their lead wires are prone to breakage if the probe is subjected to inadvertent mechanical shock, or is even rapidly drawn through an unusually rough portion of tube. While strain gauge-type probes are capable of detecting the presence of ovality in such tubes (which in turn indicates if the tube has been stressed as a result of intense, localized pressure), the flaw resolution of many of these types of profilometers is relatively coarse. If the flaw resolution is increased by the addition of more spring fingers and strain gauges around the circumference of the probe, the gauges must be made even smaller, which increases the fragility of the device further.
Eddy current type probes can also suffer from excessive fragility in designs where a tiny coil resiliently engages the interior of a wall in wiping contact. While some of the better probe designs overcome this defect by either putting the eddy current probe in a self-lubricating plastic (which is subject to wear), or by attaching the coil to the back of a stylus which resiliently engages the inner tube wall as the probe is translated therein, none of these designs, is capable of accurately resolving tube ovality, or measuring deposits on the ID.
Furthermore, measurement of a magnetite deposit layer is made more difficult because the magnetite layer has variable physical properties that affect current methods, such as conventional eddy current. The magnetic permeability and porosity of magnetite are the primary sources of these problems.
An eddy current method of measuring the magnetite layer has been developed in the past by the present Applicant. The method comprises use of an eddy current bobbin probe, excited by conventional means with a single high frequency, that records the change in the signal referenced from a portion of the tube lacking magnetite deposits. This method was developed using tube samples pulled from the field to establish a relationship between thickness and voltage. The measurements are then based on an assumed value for permeability, obtained from these pulled tubes. Thus, if the magnetic or physical properties of the layer change from tube to tube, then the eddy current response will differ, resulting in less accurate thickness estimates. Any variations in permeability of the magnetite found in the tubes evaluated in the field would cause significant errors in the estimations of thickness. With the eddy current bobbin probe technique it is not possible to separate thickness and permeability effects.
Another measurement method, the Oxiprobe™, uses the mass of the loading and the area cleaned to derive a value for the thickness [Gonzalez, F., Brennenstuhi, A. M., Palumbo, G. and Dyck, R. W., “Steam Generator Primary Side Fouling Determination Using the Oxiprobe Inspection Technique”, Fourth International Conference On CANDU Maintenance, Toronto, 1997 Nov. 16-18]. With this method, the magnetite was also assumed to have a certain, consistent, density.
U.S. Pat. No. 4,876,506, describes an apparatus and method for inspecting the profile of the inner wall of a tube employing a wall follower and an eddy current probe. The disclosed apparatus includes (i) a cylindrical probe body that is insertable within the tube, (ii) a probe assembly disposed within the probe body that includes an eddy current sensing coil and a copper plate which are movable with respect to one another, and (iii) a wall follower assembly including a stylus on one end and which is linked to the probe assembly on its other end for converting changes in the radius of the tube wall into changes in the distance between the eddy current sensing coil and the copper plate.
The probe of U.S. Pat. No. 4,876,506 is not an axial-scanning probe. Further, the probe cannot be extended for use for anything other than the inside diameter of the tube and, thus, cannot measure the thickness of any inside deposits on the tube wall. Third, the eddy currents from this module are coupled with the actual tube itself.
There remains a need for an apparatus and method that accurately and reliably measures deposits, such as magnetite, on the ID of steam generator tubing.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.